On Sat, 22 Jul 2006 23:03:29 GMT, "Robert Haston"
wrote:

I'm moving a 162 MHz (marine band ship tracker - AIS) receiver to within a
couple feet of its quarter wave ground plane antenna.


Is there a certain cable length I should use to tune for maximum gain?


There are some very common misconceptions around regarding
cable length impedance matching.

Ideally your antenna has 50 ohms, the cable has 50 ohms,
the transmitter has 50 and there is perfect matching all the time,
at any cable length.

Side note:
Did you note I left out the receiver?
Does anybody know the input impedance of his receivers in use?
How would you measure the input impedance?

Back to the cable lenght, may I spill a few words.
One may use the feed cable to match a non 50 Ohm antenna
to the 50 ohm transmitter, but that works sufficiently only for
a very narrow frequency range. Some people do it on shortwave
and they switch different lenght cable loops with a relais
to cover an entire SW band. Works fine, if no antenna tuner
is available. On the UKW bands this is rather unusual,
and normally not necessary. Go and tweak the antenna
lenght and the radials to optimum SWR. The cable tuning
method may fail with broadband antennas, of course.

w.